The present invention relates in general to a heat recovery system using a two-phase fluid circuit. More particularly, the present invention relates to a heat recovery system for recovering the heat extracted from the air drawn from a high-temperature compression stage of the engine.
In military and civil aircrafts the air-conditioning system uses typically as an air source the air drawn from a compression stage of the engine operating at a high temperature (in the region of 500-600° C.). This air, before being sent to the user appliances of the aircraft air-conditioning system, must be cooled by heat exchange with dynamic air supplied from the external environment. For this purpose, according to the prior art, it is envisaged using a primary heat exchanger, commonly known as a precooler, through which the heat extracted from the hot air drawn from the engine compressor is discharged into the external environment, without therefore being recovered.
U.S. Pat. No. 4,516,631 describes a cooling system, intended in particular for cooling a nozzle for supplying fuel or air to a combustion chamber, using a so-called “heat pipe”. According to this known solution, the cooling system comprises an evaporator device which is arranged around the nozzle and is connected by means of a duct to a tank containing a two-phase fluid, the evaporator device, the duct and the tank forming together a closed circuit. The evaporator device comprises a casing having an inner cylindrical wall in contact with the wall of the nozzle and an outer cylindrical wall which encloses, together with the inner cylindrical wall, a cavity. A dividing wall is arranged between the inner wall and the outer wall of the casing of the evaporator device, said dividing wall being formed as a metal cylinder which is perforated along at least one section of its length and dividing radially the cavity into an inner cavity extending between the inner wall and the dividing wall and into an outer cavity extending between the dividing wall and the outer wall. An element of porous material which extends between the inner wall and the outer wall of the casing is placed at one end of the cavity. The operating principle of this known cooling system is as follows. The working fluid in liquid phase supplied from the tank enters into the evaporator in an axial end zone of the outer cavity and flows axially along this cavity until it reaches the element of porous material. The working fluid, still in liquid phase, then passes from the outer cavity to the inner cavity, flowing by capillarity through the element of porous material. The working fluid then flows axially along the inner cavity, receiving heat from the nozzle and therefore passing from the liquid phase to the vapour phase. Along the last section of the inner cavity the working fluid in vapour phase releases heat to the outside through a finned heat dissipator arranged around the inner cavity and then flows out the evaporator in liquid phase and back into the tank.
The cooling system known from document U.S. Pat. No. 4,516,631 does not have the function of recovering heat from the nozzle so as to allow use of the recovered heat for given purposes, for example for heating the fuel or for de-icing function, but simply serves to extract heat from the nozzle in order to cool said nozzle, dispersing the extracted heat into the external environment. Moreover, this known system is extremely inefficient, because of the head losses which occur both in the flow along the outer cavity and in the flow along the inner cavity, and also because the fluid in vapour phase which flows along the inner cavity from the end where the element of porous material is placed to the opposite end inevitably releases heat to the fluid in liquid phase which flows in the opposite direction along the outer cavity. Because of its low efficiency, it would not be possible to employ such a system in order to use advantageously, even at a distance of several meters from the nozzle, the heat recovered from the nozzle.